As shown in FIG. 1, in a plate microlens 1, gradient index lenses 3 are buried integrally in a flat transparent base or substrate 2 of glass or plastic. One refractive surface of each lens 3 is a flat surface coinciding with the surface of the base 2. Each lens 3 has an optical axis along a normal to the surface of the base 2. The refractive index of each lens 3 along its optical axis is maximum at the surface and gradually decreases with an increase in depth. The refractive index of each lens 3 in a direction perpendicular to the optical axis is also maximum at the center and gradually decreases toward the periphery.
The shape (as viewed from the top) of each lens 3 is circular as shown in FIG. 2 or linear as shown in FIG. 3.
A typical method for manufacturing a plate microlens 1 as described above will be described below. As shown in FIG. 4, the surface of a glass base 2 is covered with an ion-permeation preventive mask 5 such as a metal thin film. Small apertures 6 of, e.g., circular shape corresponding to the shape of desired lenses 3 are formed in the mask 5. The surface of the base 2 having the mask 5 thereon is dipped in a molten salt 7 containing cations such as T1 ions which increase the refractive index of the base glass. Then, the cations in the molten salt 7 diffuse into the base 2 through the apertures 6 in the mask 5. After ion diffusion is performed in this manner for a predetermined period of time, an ion concentration profile is obtained below each aperture 6 in the base 2, wherein the ion concentration is maximum near the aperture 6 and decreases toward deeper and peripheral portions of the prospective lenses 3. Thus, portions having the gradient refractive index as described above, i.e., lenses 3 are formed.
In such a conventional plate microlens 1, as shown in FIG. 5, a condition d/a=1.0 (where 2a is the diameter of the lens 3 in the surface of the base 2 and d is the thickness of the lens 3), i.e., the cross-section of the lens 3 is almost completely semicircular. When such a lens has a complete semicircular cross-section, it is believed to have best optical characteristics. In view of this, efforts have been made to date to obtain lenses having ideal semicircular cross-sections.